The present invention relates to an X-ray transmitting membrane for a mask used in X-ray lithography or, more particularly, to an X-ray transmitting membrane having excellent transmissivity to visible light, resistance against irradiation with high energy radiations and resistance against chemicals and moisture as well as smoothness of the surface and free from pinholes and other defects so as to be useful as a mask in the X-ray lithography and a method for the preparation thereof.
Along with the remarkable trend in recent years toward finer and finer patterning on semiconductor silicon wafers in the manufacture of various kinds of semiconductor devices such as LSIs, VLSIs and the like, the conventional photolithographic patterning method by using ultraviolet light is under continuous replacement with the so-called X-ray lithography in which the resist layer on a semiconductor silicon wafer is irradiated pattern-wise with X-rays through a so-called X-ray mask bearing a desired pattern impermeable to X-rays as formed on an X-ray transmitting membrane.
The X-ray transmitting membrane for an X-ray lithographic mask should satisfy several requirements including that:
(1) the membrane is made from a material which is stable and resistant even against a large dose of irradiation with high-energy beams such as X-rays as well as high-energy electron beams and synchrotron radiations;
(2) the membrane should be highly transparent with at least 50% transmission of visible light so as to enable exact alignment of the X-ray mask on the silicon wafer with high precision even when the membrane has a thickness large enough to ensure good mechanical strengths;
(3) the membrane should be chemically stable and resistant against chemicals and moisture so as not to be damaged or attacked in the process involving etching, washing and the like sometimes using strong chemicals; and
(4) the membrane should have a smooth surface and be free from pinholes and other defects.
Several materials have been proposed in the prior art as a material of membranes for X-ray lithographic masks including boron nitride BN, silicon nitride Si.sub.3 N.sub.4, silicon carbide SiC and the like but none of them can satisfy all of the above mentioned requirements altogether since each of them has its own merits and demerits. For example, these prior art membranes are insufficient in the resistance against high-energy beam irradiation as the most important property which the membrane should have. The reason for their poor stability against radiations is presumably as follows. While these membrane are formed by the so-called chemical vapor-phase deposition (CVD) method or, in particular, by the plasma CVD method, namely, the membrane formed by the CVD method usually contains a large amount of hydrogen atoms sometime reaching 10 to 20 atomic % or even larger and these hydrogen atoms are suspected to be responsible for the instability of the membrane under irradiation with high-energy beams.
For example, X-ray transmitting membranes of silicon nitride are prepared by the CVD method using trichlorosilane and ammonia as the reactant gases under reduced pressure of 0.1 to 1 Torr at a temperature of 700.degree. to 900.degree. C. The silicon nitride membranes prepared by the above mentioned CVD method were considered in the prior art to be substantially free from hydrogen atoms because no absorption bands assignable to the bonds of Si-H, N-H and the like could be found in the infrared absorption spectrum obtained by the FT-IR method and the like. Despite the seeming absence of hydrogen atoms, the silicon nitride membranes in the prior art are subject to deformation when they are used as an X-ray lithographic mask and irradiated with high-energy beams. It is a presumable reason therefor that, although the membrane is free from hydrogen at least by the infrared absorption spectrophotometric analysis, a considerable amount of hydrogen, which can be detected by a more sensitive analytical method, is contained in the membrane to play an adverse role under irradiation with highenergy radiations. It is also presumable that such a large content of hydrogen in conventional silicon nitride membranes is caused as a result of the plasma CVD method which is conducted usually at a relatively low temperature of up to 800.degree. C. In fact, a silicon nitride membrane was prepared by the CVD method at 800.degree. C. or below from trichlorosilane and ammonia as the reactants and the hydrogen content in the membrane was measured by the RBS-HFS method to detect 2.0 to 5.0 atomic % of hydrogen therein although no absorption band assignable to hydrogen could be noted in the infrared absorption spectrum.